As is known, ferrules are cylindrical elements of both parts of many components for optical telecommunication systems, such as optical fibers, connector ferrules, etc. must have geometric characteristics whose values are to lie within certain tolerance intervals with respect to a nominal value. Among those characteristics, the external size, the non-circularity and the eccentricity of a central part with respect to the external surface may be cited for the components mentioned above, which form nominally cylindrical guiding structures.
When testing the structure, a check of such tolerances is necessary.
Of course, a direct measurement of the parameters of interest could be carried out by resorting to mechanical instruments or to microscopy techniques with the help of suitable scales. However mechanical instruments can be used only to determine the external dimensions and the parameters depending thereon, such as non-circularity. Moreover instruments able to precisely evaluate the dimensions involved (of the order of some hundred micrometers at most, in the case of fibers) are extremely expensive. Measurements with microscopy techniques also allow evaluation of internal parameters; however such techniques are not always sufficiently precise.
For these reasons the measurement of the geometrical parameters of a guiding structure comprising an external cylindrical part and an internal part coaxial with the external one is generally carried out by analyzing an image of the structure itself, obtained by suitable methods, such as those defined in CCITT Recommendations G.650, G.651 for the measurement of mode field (spot size) and cladding diameters, of mode field concentricity error and of cladding non-circularity in single-mode and multimode optical fibers. Owing to the analogy of the structure, some of these methods could also be used for characterizing connector ferrules, either in the presence or in the absence of a fiber fitted in the hole.
The methods based on the analysis of an image of the structure have the disadvantage that a precise evaluation of all parameters can be obtained from a single image only if the absolute values of the parameters are not considerably different. "Not considerably different" denotes here a difference of about one order of magnitude at most, as it is the case for instance of the mode field and the cladding diameters in a single-mode optical fiber (about 10 .mu.m versus about 125 .mu.m).
If on the contrary the values differ by some orders of magnitude, images with different magnifications are used to measure the different parameters with the same relative precision. The different images are generally obtained with two distinct operations and by different techniques. In this way, however, there is the risk of losing the correlation between the different parameters. This is just the situation occurring in a case of great interest for the invention. In fact, the parameters to be measured for characterizing the ferrule include the non-circularity of the external surface and the eccentricity of the axial hole, measurement of the latter parameter demanding measurement of the eccentricity of the mode field of a single-mode fiber inserted in such a hole. The mode field has, as mentioned, a size of the order of 10 .mu.m, while the external diameter of the ferrule is of the order of some millimeters. If different images are used for the mode field and the ferrule diameters the distance between the ferrule and mode field centers is difficult to precisely evaluate.